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472 Eur. J. Lipid Sci. Technol. 102 (2000) 472–486

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1 Introduction

Animal fats and vegetable oils are consumed because ofthe content of triacylglycerols, but they do not consist on-ly of triacylglycerols, at least when they are obtained fromnatural materials using industrial processing. They alsocontain several groups of accompanying substanceswhich may be useful as nutrients, but at the same timethey are either objectionable from the point of view of sensory value (they affect the taste, odor, color, and appearance) or from the point of view of functional prop-erties. The most important groups of these minor sub-stances are listed in Tab. 1. The composition of accompa-nying substances is influenced by processing methods, e.g. oilseed conditioning increases the content of phos-pholipids and other minor substances in the extracted oil.

2 Traditional refining procedures

Crude vegetable oils are only rarely used without previ-ous refining except virgin olive oil. The oldest refiningmethods were based on the removal of free fatty acids forthe following reason. In the most common European tra-ditional fats and oils, olive oil prevailed in the Mediter-ranean countries while pork lard and butter prevailed inthe Northern part of Europe. High quality products hadlow free fatty acid content, on the other hand high acidvalue was characteristic for low quality products. There-

fore, the refining plant wished to produce edible fats andoils with very low acid values, even though free fatty acidsare not objectionable, unless present in high amounts. Upto now, most customers still connect low acid value withhigh product quality.

Free fatty acids could be easily removed by washingcrude fat or oil with a solution of sodium hydroxide or sodi-um carbonate. Therefore, the process was called alkalirefining. It is still used, in a more sophisticated form, inmany factories. Alkali refining alone improved the productquality, but it was not sufficient to remove all objectionablesubstances so that other refining steps had to be intro-

Jirí Cmolíka, Jan Pokornyb

a Development Department,SETUZA A.S., Ustí nadLabem, Czech Republic

b Department of FoodChemistry and Analysis,Institute of Chemical Technology, Prague,Czech Republic

Physical refining of edible oilsCrude oils obtained by oilseed processing have to be refined before the consumptionin order to remove undesirable accompanying substances. The traditional alkali refin-ing is often replaced by physical refining in which the use of chemicals is reduced. Themost widely used method is steam refining. The crude oil quality is very important inorder to obtain high quality refined oil. Furthermore, the oil should be efficientlydegummed to remove phospholipids as well as heavy metals and bleached to removepigments. The most important step consists of the application of superheated steamunder low pressure and at temperatures higher than 220 °C. Both free fatty acids andobjectionable volatiles, formed by cleavage of lipid oxidation products, are removed. Adisadvantage is the partial loss of tocopherols. Side reactions, particularly isomeriza-tion of polyunsaturated fatty acids, should be minimized. The quality of physically re-fined oil is close to that of alkali refined oils, but losses of neutral oil are lower and theenvironment is less polluted. Among other methods of physical refining the applicationof selective membranes is promising.

Keywords: Bleaching, degumming, edible oils, membrane refining, physical refining,steam refining.

Correspondence: Jirí Cmolík, SETUZA A.S., CZ-401 29 Ustínad Labem, Czech Republic. Phone: +420 47 5292291, Fax:+420 47 529 3999; e-mail: jiri.cmolik@setuza.cz

Tab. 1. Minor substances present in crude fats and oils.

Substance Typical Deteriorating class representatives effect

Oxidation Volatile aldehydes, products ketones, hydrocarbons Off-flavors

Free fatty Saturated and Lower oxidative acids unsaturated stability,

fatty acids impaired functional properties

Phospholipids Lower oxidative stability

Pigments Chlorophylls, Lower sensory carotenoids, propertiesmyoglobin

Metal salts Iron and copper Lower oxidative compounds stability

duced (Tab. 2). Phospholipids and other polar lipids wereremoved by alkali refining with free fatty acids, but it wassoon found that it would be better to remove phospho-lipids in a separate step consisting of heating crude rawmaterial with water or an aqueous solution of phosphoricacid. The sedimented materials, called gums, could beremoved by centrifuging and used separately from freefatty acids, removed by the subsequent alkali refining.

Pigments were removed by bleaching with bleaching clayor charcoal, and volatile substances were removed bydistillation with steam at high temperatures and underreduced pressure. The resulting product was nearly colorless, bland with no appreciable flavor, and had agood stability on storage.

Alkali refining generally is used in food industry, but it has several disadvantages: 1. the procedure is ratherexpensive, 2. losses of neutral triacylglycerols occurwhich are high especially in raw materials with original

high free fatty acid content, 3. the procedure is time con-suming, 4. energy requirements are high, 5. wastes cont-aminate the environment. For these reasons, an alterna-tive way of refining was developed. Other chemical refin-ing methods, such as refining with mineral acids, are onlyextremely rarely used for edible oil production.

3 Physical refining

In methods of physical refining, free fatty acids are re-moved from crude oils by physical methods only withoutthe application of sodium hydroxide or sodium carbonate[1]. Strictly speaking, even in physical refining processingsteps are included which require chemicals. Severalphysical procedures have been developed. Most of themare only of historical interest, but many procedures maybecome useful in the future and only a few are consideredas practicable in the industry nowadays. Most technolo-gists use only one or two procedures best suitable fortheir particular conditions, not being aware of a multitudeof other procedures which could be used in other factoriesor under different processing conditions. Therefore, anoverview of methods for physical refining of crude veg-etable oils is given in Tab. 3 including those methodswhich now may be assumed as not important. Steam re-fining is the only large-scale practicable method used inthe industry nowadays, and it is mostly this method, con-nected with efficient degumming and bleaching, which isunderstood by speaking of physical refining. Neverthe-less, membrane technologies have good perspectives inthe future. Earlier methods of physical refining, with theemphasis on steam refining, were critically reviewed soonafter the introduction of the procedure [2].

Eur. J. Lipid Sci. Technol. 102 (2000) 472–486 Physical refining of edible oils 473

Tab. 2. Importance of refining steps in the case of the tra-ditional process.

Refining step Substances removed

Hydration, degumming Phospholipids, other polar lipids (gums)

Neutralization Free fatty acids, residual phospholipids, metals

Bleaching Pigments, residual soaps, and phospholipids

Deodorization Volatile oxidation products and other contaminants

Tab. 3. Methods of physical refining of crude vegetable oils.

Procedure Principle

Steam refining (proper physical refining) Removal of fatty acids and other volatiles by superheated steam at200–270 °C at low pressure after preliminary degumming and bleachingsteps.

Inert gas stripping Removal of free fatty acids in a stream of inert gas (nitrogen).

Molecular distillation Removal of more volatile components, including free fatty acids, from lessvolatile triacylglycerols at very low pressure, without application of steam.

Membrane refining Treatment of crude oils under pressure with selective membranes, permeable for free fatty acids, but not permeable for triacylglycerols.

Hermetic system Treatment of oil at high temperature and pressure, followed by centrifugationof precipitates, followed by steam refining.

Extraction with solvents Selective removal of fatty acids by countercurrent distribution between immiscible solvents, one dissolving selectively free acids, another one selectively extracting triacylglycerols.

Refining with supercritical carbon dioxide Removal of free fatty acids and other impurities using supercritical carbon dioxide.

Cholesterol removal Purification of animal fats from cholesterol, most often by steam refining.

3.1 Physical refining of low unsaturated fatsand oils

Physical refining by superheated steam is not a new in-dustrial refining method as it was introduced during theWorld War II and the following years, when new equip-ments for continuous deodorization and the use of highdeodorization temperatures were introduced. Problemsarose with the refining of vegetable oils containing higheramounts of phospholipids and being more unstable athigh deodorization temperatures. Therefore, the methodwas used mainly for less unsaturated and low phospho-lipid materials, such as lard, coconut, and palm oils. In anearlier review [3], high temperature steam refining wasclassified as a procedure for refining lard or palm oil, atleast in the United States. The same opinion was ex-pressed in a review on the progress of fats and oils refin-ing in the USA several years earlier [4]. Even in a muchmore recent review [5], physical refining is mentioned asa method mainly for refining palm oil.

Lard was a common edible fat in the USA in the postwaryears, therefore, great attention was paid to lard refiningas the US consumers prefer a bland fat, without any typi-cal lard flavor as appreciated in Central Europe. Bailey [6]proposed an apparatus for physical refining of animal fat shortening (particularly mixtures of lard and beef tallow and their fractions), consisting of a cylindrical shellwith five square, superimposed trays. The fat was lique-fied and discharged in the highest tray while the strippingsteam was introduced countercurrently. At the proposedtemperature of 175 °C the free fatty acid content de-creased only moderately. As lard was rather unstable dur-ing storage, the physical refining of lard was realized afterheating with creosot bush leaves [7] containing nordihy-droguaiaretic acid (rather efficient, but not allowed anylonger for use in food) to improve the stability. The de-odorization temperature was 400 to 500 °F (205–265 °C ),when the acid value had to be decreased at the sametime, and only 350 °F (about 193 °C ), if the acid valuewas no crucial quality indicator. Steaming for 3–4 h was proposed as sufficient. In a postwar patent to East-man Kodak Co. [8] the deodorization temperature of80–280 °C was proposed at 10 mm Hg (1.3 kPa). In alater paper [9] the main purpose of steam stripping of lardwas the removal of free fatty acids. A special deodoriza-tion unit was patented for the physical refining of lard [10].

Coconut and palm kernel oils, extracted from palm seeds,have very low unsaturation and low phospholipid content.Free fatty acids have shorter hydrocarbon chains and,thus, a higher volatility than other seed oils. Therefore,they could be easily refined by superheated steam. Therefining was very important as medium chain length fattyacids are sensorically more objectionable than higher fat-

ty acids. Palm seed oils are low unsaturated so that theirfatty acids are relatively resistant to high temperatures.

Earlier literature was reviewed by Tandy and McPherson[11]. The quality of physically refined coconut, palm ker-nel, and palm oils was compared with that of the respec-tive alkali refined oils and substantial differences were ob-served [12].

In an early patent [13] centrifugal force was applied tocontrol the flow rate of coconut oil through the neutraliza-tion/deodorization column. The free fatty acid content was reduced from 0.6 to 0.05% at the recommended temperature of 205–235 °C and the pressure between0.5–1.1 kPa [14]. Substantial removal of free fatty acidswas obtained by steam deodorization of coconut oil at150–170 °C and 1.1–2.6 kPa for up to 6 h. Reduction ofthe acid value was only slight in polyunsaturated oils,such as sunflower and soybean oils, under the same con-ditions [15]. Palm kernel oil has a similar composition tothat of coconut oil, so that it is easily physically refined un-der similar processing parameters. Phosphoric acid wasrecommended [16] as a suitable pretreatment to removephospholipids.

Palm oil, which is a pericarp oil, has another compositionthan palm seed oils containing only traces of mediumchain length fatty acids. It is far more important for edibleuses than coconut or palm kernel oils, therefore, its refin-ing was studied very early. The main reason is that it oftencontains high amounts of free fatty acids. In case of highacidic oils, physical refining may substantially reduce therefining losses of neutral oil. A typical early procedure wasreported by Bloemen [17] using a deaerator and twosteam refining columns. The second one operating at0.5–0.7 kPa. A sample of palm oil containing 12% free fatty acids could be refined at 0.8 kPa to obtain refined oil containing 1.3% free fatty acids at 220 °C and 0.5%free fatty acids at the refining temperature of 230 °C. Ofcourse, much lower free fatty acid contents are requirednow.

Bernardini [18] recommended a packed column for phys-ical refining of palm oil after degumming to 2–3 mg/kgphosphorus. The column height was 4–5 m with 250 m2

of packing surface per 1 m3. The column could be usedfor deacidification of high acidity olive oil (up to 30% freefatty acids).

The physical refining process of palm oil has been sub-stantially improved since then, usually including thedegumming step, but the high temperature process re-sults in a destruction of carotenes, and, thus, in a deterio-rated nutritional value and even appearance for somepurposes. Therefore, lower deodorization temperatures

474 Cmolík et al. Eur. J. Lipid Sci. Technol. 102 (2000) 472–486

may be useful to prevent the heat bleaching even whenfree fatty acids are only partially removed. For instance, atwo-step process was reported [19] in which palm oil,acidified to remove phospholipids and other gums, wassteam stripped at temperatures up to 200 °C followed bythe second step of heating up to 180 °C. This procedure issuitable to obtain oil with agreeable flavor according tothe patent claims. Higher temperatures are used now,e.g. steam stripping at 240–270 °C and 0.25–1.32 kPa [5].Two modern deodorization devices, i.e. Mellapak® andthe more recent Optiflow®, were discussed [20] whichguarantee satisfactory mass transfer and a substantiallyimproved steam distribution.

Red palm oil has been introduced more recently into themarket as the red coloration is agreeable to the consumerand a high carotene content is desirable because of itsnutritional value and antioxidant activity. A molecular dis-tillation unit was proposed [21] in which the free fatty acidcontent was reduced to below 0.1%, while more than 80%carotenes and tocopherols were retained.

In a film column cocoa butter, which belongs to low un-saturated and easily refined fats, could be efficiently neu-tralized/deodorized [22]. The molecular distillation is ex-pensive (it will be discussed later), but for pharmaceuticalpurposes the process is acceptable. The procedures ofsteam refining recommended for palm oil, which aremuch cheaper, could be used for cocoa butter, too.

3.2 Physical refining of polyunsaturated oilsThe application of physical refining requires a change inother refining steps, too. Therefore, the degummingsteps, and to lesser extent the bleaching steps, will bediscussed here as they are of crucial importance, eventhough they do not belong to physical refining in its narrower sense.

3.2.1 Degumming and bleaching for physical refiningAfter the introduction of physical refining into the industry,the process was applied to a variety of fats and oils, es-

pecially in Europe [3], where the industry had to processseveral oilseeds in a plant. In their comparison of qualitycharacteristics of physically refined oils Strecker et al. [12]discussed corn, sunflower, canola, soybean, peanut, ricebran, safflower oils, and oils derived from palms. As ordi-nary degumming process mainly water degumming wasused at first, but it was soon found that the quality and oxidative stability of such a physically refined oil wasrather low due to high residual phospholipids and a heavymetal content. New degumming methods had to be developed [23]. Dijkstra [24] defined several degummingprocesses which are used or might be potentially useful(Tab. 4).

It is relatively easy to remove hydratable phospholipidsfrom crude oils as they readily absorb water thereby becoming insoluble in oil. A problem is the removal ofnonhydratable phospholipids, mainly calcium and magne-sium salts of phosphatidic acids. It would be preferable tokeep the content of phosphatidic acids low in seeds. Un-ripe, damaged, or moist seeds, which have to be dried be-fore processing, are generally high in phosphatidic acidsalts so that they are refined with difficulty only. Therefore,such seeds are unsuitable as raw materials for physicallyrefined oils.

In ripe seeds phosphatidic acids are mainly produced byenzymatic hydrolysis during seed crushing. Therefore, itis advantageous to deactivate phospholipases in a veryshort time. A suitable procedure [25] is to treat oilseedswith superheated steam (heated to 150–170 °C ) for a fewseconds to increase the moisture content from about7.5% to 17% and then to dry the material back to 5–6%.The whole process takes about 20 s. In such a short timephospholipids do not hydrolyze appreciably into phospha-tidic acids, but the enzymic activity of phospholipases isdestroyed by the heat. Dry flakes are cooled to 90–95 °Cin about 40 min and the final moisture is about 6%. Undersuch conditions the contents of phosphatidic acids andtheir iron, calcium, and magnesium salts are only about25–50% of the amount obtained by conventional process-es.

Eur. J. Lipid Sci. Technol. 102 (2000) 472–486 Physical refining of edible oils 475

Tab. 4. Degumming processes before the physical refining of vegetable oils.

Type of degumming process Principle of the procedure

Water degumming Treatment of crude oil with hot water

Acid degumming Treatment of crude oil with phosphoric acid or citric acid

Acid refining Water degummed crude oil is treated with an acid, which is then partially neutralizedwith alkali

Dry degumming Acid degumming with very small amount of water, combined with bleaching

Enzymic degumming Modification of phospholipids with enzymes to facilitate the hydration

Membrane degumming Passage of crude oil through a semipermeable membrane retaining phospholipids

A similar process was covered by a patent [26] consistingof forming a slurry from oilseeds and oil, deactivating en-zymes by heating to 99 °C under reduced pressure, andpasteurizing the slurry. The resulting oil has a low contentof phosphatidic acid salts and may be directly used forphysical refining. Another suggestion was to extract finelyground oilseeds with hexane followed by acid degummingof the miscella [27]. The process was tested on plantscale, but the difficulty is to remove the solvent quantita-tively from the miscella.

The time between flaking soybeans or other oilseeds andtheir extraction should be very short, and the temperaturehigh [28]. Such pretreated oilseeds produce crude oil witha lower content of nonhydratable phospholipids. The pas-sage of oilseed material through an expander is advanta-geous as the lipases are deactivated almost immediatelyby expander cooking.

A variety of degumming procedures has been introducedinto the industry in the last twenty years [23] more or lesssatisfying requirements for physical refining. The com-mon experience is to obtain a good oil quality throughphysical refining. The phospholipid content of the oil mustbe sufficiently low (less than 5 mg/kg phosphorus beforestripping and less than 20 mg/kg phosphorus before thebleaching step). This requires not only the removal of hydratable phospholipids as is usually achieved by waterdegumming, but also the removal of nonhydratable phos-pholipids (calcium and magnesium salts of phosphatidylethanolamine and phosphatidic acids).

Since about 1980 steady introduction of physical refiningprocesses was observed by oil technologists, which re-quire more efficient degumming processes. First degum-ming technologies allowing physical refining, involved, forinstance, dry degumming of crude oil followed by physicalrefining as practised for palm oil and acid degumming ofcrude oil, either water degummed or not, followed by drydegumming and physical refining as practised for rape-seed oil.

During the water degumming process hot water is mixedinto warm oil (70–90 °C). Hydratable phospholipids absorb water to agglomerate into a gum phase, which is,after a certain holding time, separated in separators ordecanters. The procedure may be sufficient for sometypes of vegetable oils, e.g. 99% of phospholipids are removed by heating oil to 80–85 °C with water and thenfiltering at 55–60 °C [29]. However, the water degummingdoes not destroy nonhydratable salts of phosphatidicacids, therefore, it is not sufficiently efficient in most cases for the preliminary purification of oils before physi-cal refining.

For the acid degumming originally phosphoric acid wasused as it forms nondissociable phosphates with calcium,iron, or magnesium ions and is much cheaper than poly-valent organic acids. The use of 0.05–0.20% phosphoricacid was found satisfactory for the degumming of palm oil[20]. It was found efficient for the degumming of palm ker-nel oil [16], but not for most other vegetable oils. For thedegumming of soybean oil the application of phosphoricacid and water was recommended [30], first with a con-centrated acid and then with diluted acid. A treatment withphosphoric acid was proposed for the degumming of soy-bean oil prior to physical refining in combination with acoacervation agent [31], such as hydrated phospholipids.

Citric acid is an alternative to phosphoric acid. It is moreexpensive than phosphoric acid, but the removal of non-hydratable phospholipids is more efficient. Heating ofcrude rapeseed oil with 0.1% citric acid to 70 °C with re-moval of the precipitate by centrifugation resulted indegummed oil containing only 2 mg phosphorus and0.3 mg iron in 1 kg oil [32]. Acetic anhydride was foundsimilarly efficient as citric acid for the degumming of soy-bean oil [33]. Sodium citrate was reported as more effi-cient for removing calcium, magnesium, or iron from non-hydratable phosphatides than free citric acid [34].

Water refining or refining with phosphoric or citric acidsalone is nowadays no longer generally considered as a sufficiently effective degumming step before physical refining and several more sophisticated procedures havebeen developed.

In the dry degumming process, crude oil is treated with anacid (usually 75–80% phosphoric acid) to decompose themetal salts of acidic phospholipids. The acid is dispersedin the hot oil (80–100 °C) in amounts of 0.05–1.20%.Some water may be added to enhance bleaching efficien-cy and bleaching earth is added (about 1–3% on the ba-sis of oil). The bleaching procedure is carried out at 120 or140 °C under reduced pressure. The bleaching earth isthen removed by filtration. In the dry degumming processa part of the acid activated bleaching earth can be re-placed by synthetic silica hydrogel.

The Total Degumming Process includes both aciddegumming and bleaching steps [35]. The amount ofbleaching earth is of crucial importance as the degummedoil should not contain more than 0.2 mg/kg iron, other-wise, it would be unstable during storage. The residualphosphorus content should not exceed 10 mg/kg, but wewould recommend 5 mg/kg as the limiting value. The useof bleaching earths Tonsil ® Optimum or Tonsil ® Supremewas recommended in the bleaching step after citric aciddegumming as suitable for sunflower, corn, soybean,peanut, and rice bran oils [36]. In another patent applica-

476 Cmolík et al. Eur. J. Lipid Sci. Technol. 102 (2000) 472–486

tion [37] crude oil is water degummed, then degummedwith an acid, and a highly acid bleaching earth is added.The use of a combination of 3% bleaching clay and 0.3%activated carbon was recommended for the purification ofedible oils previously degummed with phosphoric acidand water [38]; heating to 110 °C for 10 min in vacuumwas recommended. Trisyl ® Silica (a silica gel adsorbent)was found useful for the removal of iron and for decol-orization of rapeseed and sunflower oils [39] and a mix-ture of acidic synthetic adsorbents with activated clay forrapeseed and soybean oils [40]. If the degumming andbleaching steps are separated in the physical refiningprocess, the degummed oil for the subsequent bleachingstep should not contain more than 30 mg phosphorus perkg [41], otherwise the oil cannot be efficiently bleached.

Advantages of the different procedures mentioned abovehave been combined in the acid degumming process (superdegumming). In the acid degumming process acombination of degumming acid and water is used to decompose the bivalent metal ion complexes with phos-pholipids. The process has been critically reviewed by An-derson [42] who also discussed the aspects of lecithinquality. A similar procedure of acid refining which con-sisted of adding water, heating, and centrifuging was suc-cessfully used for the physical refining of sunflower oil[43].

In the acid degumming process, introduced by Unileveras the superdegumming process, a concentrated (e.g.50%) solution of citric acid is added to crude oil, either water degummed or not, and heated to 70 °C . After a reaction period the mixture is cooled to below 40 °C andwater is added. During a further holding period liquidphospholipid crystals are formed. These crystals are thenremoved, but the oil mixture must be preheated beforebeing brought into the separator because of its high vis-cosity at lower temperatures. Optionally, a specially prepared easily hydratable modified lecithin agent can bemixed into the crude oil to improve hydratability of theoriginally present phospholipids.

Recently, Unilever has extended the superdegummingprocess to the unidegumming process which consists ofseveral steps starting with the standard superdegummingprocedure which is followed by a further cooling step, theaddition of water or alkali, a further agglomeration stageat 25 °C, followed by a heating and a final separation step.

Acid degumming processes usually lead to lower residualphosphorus and iron contents, than those of waterdegumming. Acid degumming processes are preferred forprocessing rapeseed and sunflower oils the gums ofwhich are commonly mixed with extracted meal for feed-ing purposes so that the quality of the gums is not crucial.

The lecithin quality from the acid degumming is not equiv-alent to that of lecithins obtained by traditional simplerdegumming processes [42] which may affect their appli-cation for various purposes.

Recent developments involve the acid refining of crudeoil, either water degummed or not, where the oil is treatedwith a degumming acid that is partially neutralized bysodium hydroxide or caustic soda. Such an acid refinedoil can subsequently be bleached and physically refined.According to Ajana and Hafidi [44] the acidified oil is neu-tralized and washed. No phosphatidyl choline and phos-phatidyl ethanolamine remained in the refined rapeseedoil after neutralization, and the remaining phosphatidicacids were removed by washing. In the case of soybeanoil only phosphatidyl choline was removed by neutraliza-tion while phosphatidyl ethanolamine was completelyeliminated by washing only, and the phosphatidic acidsremained even in the washed oil. After Nock [45, 46]crude rapeseed oil was acidified with citric acid, about50–75% acids were neutralized with alkali, and acid acti-vated clay was then added to finish the degumming. Thephosphorus content decreased from 2.4 g/kg to 5 mg/kg.The suspension was filtered afterwards [46]. The heavymetal content can be reduced by addition of 0.1–0.5%complexing agents [47].

On the basis of preliminary studies the acid refiningprocess was developed as follows: Nonhydratable phos-pholipids first are decomposed by a sufficiently strongacid. After this acid treatment, instead of diluting thedegumming acid with water as in the acid degummingprocesses, the hydratability of phospholipids is achievedby the addition of a base, such as sodium hydroxide,caustic soda, or sodium silicate. To prevent soap forma-tion the degumming acid is only partially neutralized. On a large scale, for instance, the Vandemoortele TotalDegumming Process (TOP) was developed for vegetableoil refining. Phosphoric acid being nontoxic, food grade,and cheaper than citric acid is preferred as a degummingacid. Presently, an improved degumming procedure iscalled “TOP-dry Process” because of its reduced waterconsumption and small effluent volume. Degumming acid is intensively mixed into the heated oil and after ashort contact time (2–3 min) partially neutralized withcaustic soda. Gums are separated in the first centrifugalseparator. Washing water is mixed into an oil stream before it is passed to the second centrifugal separator(clarifier) [24].

The above degumming procedures are suitable for plantscale processes, but the choice of the best procedure depends on the raw oil quality, plant equipment, and requirements for refined oil.

Eur. J. Lipid Sci. Technol. 102 (2000) 472–486 Physical refining of edible oils 477

A soft degumming process developed by Tirtiaux [48] consists of mixing the crude oil, either water degummedor not, with a reactive aqueous solution of a complexingagent (EDTA) which forms very stable chelate complexeswith polyvalent metal ions.

Elegant procedures are membrane degumming tech-niques [49] which replace the subsequent alkali refiningand often combine the degumming step with the bleach-ing step. Semipermeable membranes are used which allow the passage of substances of either defined polarityor defined molecular size. It is advantageous to refine oil in miscella using a polyimide membrane before the removal of extraction solvent [50] as the viscosity of thesolvent free liquid phase would be much higher than thatof a miscella. The membrane refining of crude soybeanand rapeseed oils was performed with multilayer NTGS-1100 and NTGS-2100 membranes at a pressure of 3 MPaand at a temperature of 40 °C [51]. In the presence of water phospholipids form micelles of the size of about20,000 Da which do not pass the membrane pores, e.g. of a DS-7 membrane, when the phosphorus content isclose to zero in the permeate [52]. Thereby the loss ofneutral oil during the degumming is reduced substantiallyby up to 75%. Membrane degumming was reviewed [53]and pilot plant experiments were compared with laborato-ry experiments. Some problems still exist, such as plug-ging or recycling, and the mass transfer through mem-branes should be improved, too [54]. The membranedegumming is too expensive for industrial purposes, but itwill be developed in a few years into a technology whichcould be used at least for pilot plant degumming of spe-ciality oils.

Low temperature degumming [55] requires only 40–45 °C.Sunflower oil was successfully degummed using electricseparators. The process capacity was 10 or 50 t/h withthe residual phospholipid content of 0.14–0.20% [56].Moulton and Mounts [57] suggested ultrasonic degum-ming of crude soybean oil with the addition of smallamounts of degumming agent. The authors claimed thatthe process was very fast and 90–99% of phospholipideswere removed.

Supercritical carbon dioxide degumming was found effi-cient, but it is still too expensive for edible oils. However,it may be used for speciality oils and for pharmaceuticalpurposes. At 55 MPa and at 70 °C the phosphorus con-tent in crude soybean oil was reduced from 620 mg/kg toless than 5 mg/kg [58]. Citric acid may be added to bindiron into complexes. A similar process (at 20 MPa and47 °C) was reported for crude peanut oil [59]. Carbondioxide is selective as a solvent not only for the removal ofphospholipids but also of pigments, trace metals, and freefatty acids. In a two-column pilot plant, a mixture of car-

bon dioxide and propane was used at 12 MPa to removeiron and most phospholipids (the residual contents of lessthan 0.02 mg/kg and 0.5 mg/kg, respectively). The secondcolumn was used for partial removal of free fatty acids[60]. Refining in a silica gel packed column was suitablefor the removal of phospholipids and pigments using onlycarbon dioxide and without hydrocarbon solvents [61]. Asupercritial carbon dioxide process [58] could removephosphorus (from 620 to 5 mg/kg) and all pigments fromcrude soybean oil. The process was efficient in removingcarotenes from palm oil and decreasing the content of polar lipids [62]. The supercritical carbon dioxide de-gumming and bleaching was found to be successful evenfor corn oil [63]. High costs will probably not allow widerapplications of the process though.

A modern method for vegetable oil degumming is the enzyme catalyzed degumming. Lurgi AG developed theEnzyMax process using phospholipase A2, which re-moves the fatty acid bound in the sn-2 position, resultingin the formation of the respective lysophospholipids whichare insoluble in oil. They can be easily separated, e.g. bycentrifuging, in the course of physical refining [64]. A sim-ilar process was patented by a Japanese team [65]. Anearly experience with the EnzyMax process was reviewedby Dahlke and co-workers [66, 67, 1], who reported theresidual phosphorus content of 2 mg/kg. In spite of beingstill under development, the enzymatic degummingseems to be a suitable plant scale technology dependingon low cost and effective enzyme preparations.

The numerous new degumming procedures, which havebeen developed in the last 15 years, enable us to produceoil with such a low content of phospholipids and other im-purities that these oils can be physically refined withoutdifficulties. Further development of several processes canbe expected in the near future.

3.2.2 Deodorization/deacidification process

When low unsaturated fats and oils started to be physi-cally refined successfully, similar procedures were stud-ied for the refining of polyunsaturated oils, of course, un-der the conditions available at that time. The main differ-ence between the conditions used for deodorization ofalkali refined soybean oil and deodorization/deacidifica-tion was the use of high temperatures – between 220–260 °C [68–70]. Both chemical and physical refining pro-cedures were compared in several reviews [71–74].

Soybean oil was a subject of intensive study because ofits importance. Twenty years ago, the physical refining ofsoybean oil was already sufficiently developed to allowcompetition with alkali refining [75, 76]. The process wasparticularly suitable for crude soybean oils of relativelyhigh quality processed in the soya producing regions. A

478 Cmolík et al. Eur. J. Lipid Sci. Technol. 102 (2000) 472–486

deodorization temperature of 240 °C, duration of 3–5 h,and pressure of 0.4–0.6 kPa were recommended, but theprocedure was applicable only for crude oils containingup to 1% free fatty acids. Removal of free fatty acids,which are relatively less volatile then sensorically activelipid oxidation products, required carefully optimized de-odorization conditions [77]. Quality parameters of physi-cally refined oils were similar to those of alkali refined oils[78]. The most important condition for the production ofhigh quality, physically refined soybean oil was a betterequipment and development of engineering aspects ofthe process [2]. Therefore, only laboratory experimentswere published from Russia [79] where such an equip-ment was not available at that time.

It was soon found that the poor quality of physically re-fined oils was mostly due to an unsatisfactory quality of oilsupplied for deodorization, in particular due to an insuffi-cient removal of phospholipids and iron [80, 81]. This in-formation caused the development of an improveddegumming. In a review published in 1983 the importancefor oils of a better quality for deodorization was empha-sized [82, 83], which stimulated the introduction of su-perdegumming. The upper temperature limit was fixed to250 °C and the time limit to 2 h, otherwise, the quality of refined oils deteriorated [84]. Even at the end of thedecade [85, 86] physical refining was questionable be-cause of an insufficiently guaranteed shelf life, probablydue to the uncomplete removal of phospholipids and iron.These opinions stimulated a further improvement of thedegumming process as discussed above.

Sunflower is the main oilseed in Russia, Ukraine, andBalkan countries. Therefore, great attention was paidthere very early to their physical refining and it was dis-cussed in early reviews published in Bulgaria [87] and inRussia [88]. The final free fatty acid content of physicallyrefined sunflower oil was 0.05–0.13% which was consid-ered very good in Russia at that time. The quality wasrapidly improved further [89] after introducing more so-phisticated degumming so that the quality of physically refined oil was the same as that of chemically refined sunflower oil. The temperature was 200–240 °C at 0.25–0.50 kPa [90]. Higher yields of physically refined oils werereported as the most important advantage of physical re-fining which reflected the emphasis of the Russian gov-ernment on quantity instead of on quality. A degummingprocess specific for sunflower oil was developed in Rus-sia [91], and another specific procedure issued from Hun-garian research plants [92]. The quality of physically re-fined sunflower oil soon reached that of the alkali refinedoil in Hungary [93].

Rapeseed oil is not easily refined by deodorization/deacidification because of the relatively high content ofthermolabile linolenic acid. Therefore, physical refining

was not introduced for canola oil until the nineties even inCanada [94]. Some experiments started in the meantimenot only in Western Europe but also in Central Europeanand Baltic countries [95]. Results of plant scale produc-tion in the Czech Republic showed [96] that a physicallyrefined rapeseed oil had the same quality as an alkali re-fined oil.

Crude rapeseed oil, produced by combination of expellerpressing and hexane extraction of low glucosinolate zeroerucic winter rapeseed of Czech origin, is degummed according to the Unilever procedure of superdegummingusing citric acid (Cmolík et al., unpublished results). Superdegummed rapeseed oil (phosphorus content lessthan 30 mg/kg) is bleached using a special bleaching pro-cedure consisting of two steps: 1) acid conditioning withan aqueous solution of citric acid followed by 2) bleachingwith acid activated bleaching earth under reduced pres-sure. The phosphorus content in the bleached rapeseedoil is usually less than 1 mg/kg. For the deodorization, asemicontinuous tray-type Lurgi deodorizer is applied (capacity about 10 t/h). The original design of the Lurgideodorizer was modified by the manufacturer for physicalrefining of rapeseed oil (improvements in the strippingsteam injection and distribution, changes in the design ofgas-lift-pumps in the main deodorization trays, and lowerabsolute pressure). The deodorization temperatures of230 or 240 °C were found to be sufficient for the removalof free fatty acids. Formation of trans fatty acids is influ-enced by the composition of unsaturated fatty acids inrapeseed oil. Oleic acid is relatively stable even at 270 °C.Therefore, the increase of monoenoic trans isomers isnegligible during deodorization. Linoleic acid is on the onehand stable at 230 or 240 °C, on the other hand at 250 °Cand particularly at 270 °C the formation of trans isomersof linoleic acid accelerated. Linolenic acid is much lessstable on heating than linoleic acid even at low deodor-ization temperatures. Earlier results confirmed that at270 °C more than 40% of linolenic acid originally presentwere transformed into various trans isomers. In otherwords, the main source of trans isomers formed in physi-cally refined rapeseed oil is linolenic acid which con-tributes more to the total trans isomer content than all other unsaturated fatty acids together. Trans isomers oflinolenic acid and those of linoleic acid are formed at a ratio of about six to one. The deodorization procedure ofrapeseed oil at lower temperatures (230–240 °C ) resultsin a significantly lower formation of trans isomers and satisfactory tocopherol retention: 1% trans isomers and72,6% tocopherol retention at 240 °C, and 0,6% transisomers and 83,6% tocopherol retention at 230 °C, re-spectively.

Contrary to large changes due to isomerization oflinolenic acid [96] the content of most polyunsaturated

Eur. J. Lipid Sci. Technol. 102 (2000) 472–486 Physical refining of edible oils 479

triacylglycerols changed only a little which is a proof of only moderate polymerization [97]. During storage for ayear, the quality of physically refined oils did not changemore than that of alkali refined oils and both types still hadcompletely satisfactory sensory values even at the end ofthe storage period [98].

Corn oil can be physically refined after degumming with water or acids, bleaching with diluted phosphoricacid, and 3% bleaching earth before steam refining. Thefree fatty acid content decreased from 1.8% to 0.010–0.023%, the phospholipid content from 440 mg/kg to10–23 mg/kg, iron from 1.4 mg/kg to less than 0.1 mg/kg.The tocopherol content decreased from 1440 mg/kg to700–950 mg/kg under deodorization conditions which stillis a fair content [99]. In Russian experience with corn oil,the free fatty acid content was reduced from 2.17–2.53%to 0.03–0.05% while the content of tocopherols de-creased from 800–1100 to 300–400 mg/kg [100].

Cottonseed oil contains no linolenic acid, but problemscould arise from its easy discoloration. It could be de-odorized at high temperatures [8]. High acidity cottonseedoil may be difficult to refine [101]. The distillates from de-odorization of cottonseed oil are a good source of sterolsand tocopherols [102].

Substantial improvement of deodorization equipment wasfound necessary for physical refining to save energy [103,104] and to improve the contact of oil with the steam [105,106]. Improved steam sparging techniques [107–110]made a decrease of the deodorization temperature possi-ble without decreasing the efficiency of the free fatty acidremoval. A more detailed discussion of engineering as-pects would require a special review.

The optimum physical refining process is not easy to recommend. As evident from the above information, theexact physical refining process should be adapted to thetype and quality of the crude oil to be refined and all para-meters of deodorization/deacidification should be opti-mized to obtain refined oils of superior quality [111]. An-other important factor is the deodorization equipmentavailable, experience of the service, and the actual re-quirements for refined oil. When the alkali refining is com-pared with physical refining, the environmental aspectswould surely favor the latter process [112]. However, thetopic should be considered from other aspects, too.Kokken [113] discussed some disadvantages of physicalrefining, such as more frequent quality defects, especiallyif crude oils of unknown quality are processed. Degum-ming is more sensitive and larger centrifuges are neededthan in chemical refining. More bleaching earth is neededwhich again requires more expensive equipment. The deodorization/deacidification is carried out at higher tem-

peratures which is not only more expensive but alsocauses isomerization of essential fatty acids and higherlosses of tocopherols. The deodorization condensatesare more contaminated with fatty acids so that they havea lower value for further processing.

Deodorization condensates from physical refining have a higher content of free fatty acids than deodorizationcondensates from alkali refined oils. The fatty acid com-position in the distillate may rather differ from that of refined oils, e.g. lower linoleic acid content was found incondensates from soybean and sunflower oils than inneutral oil [114]. Free fatty acids complicate the utilizationof unsaponifiables. For easier separation free fatty acidsmay be transformed into esters [115], and tocopherols arethen obtained by a new distillation. Immobilized lipasewas found suitable as a catalyst for the esterification offree fatty acids in distillates from soybean and canola oils[116]. Fatty acids can be converted not only to methyl esters but also to nonvolatile triacylglycerols [117].

Tocopherol losses during physical refining of vegetableoils were discussed [118]. The recovery of tocopherols isalso possible after saponification of free fatty acids [119,120]. Different phenolic acids which are found in crudevegetable oils are not found in refined oils [121]. There-fore, they or their degradation products contaminate de-odorization condensates.

Phytosterols are another commercially interesting com-ponent present in deodorization condensates from bothalkali and physical refining [122]. Physically refined oilscontain higher amounts of acylated steryl glycosides thanchemically refined oils [123]. Most steryl glycosides arealready removed by degumming, but some still remain indegummed oils before the deodorization [124]. They areinteresting because of their hypocholesterolemic activity.The concentration of sterols in the condensates is usuallyhigher than that of tocopherols, but it is still lower than that of fatty acids or neutral oil; a preconcentration step isusually necessary before their utilization [125]. In a wipedfilm short path evaporator it is possible to remove free acids and other more volatile components from tocopherols and sterols. Sterols crystallize from the tocopherol fraction [126].

3.2.3 Side reactions during physical refining

High deodorization temperature accelerates trans iso-merization reactions. The trans isomerization will be treat-ed only briefly here as the same topic, only from anotheraspect, will be the subject of a special Dossier on transunsaturated fatty acids in food lipids to be published laterthis year in the European Journal of Lipid Science andTechnology.

480 Cmolík et al. Eur. J. Lipid Sci. Technol. 102 (2000) 472–486

The phenomenon is not new, being known for nearly 30years. During the deodorization of soybean oil at240–295 °C the contents of linoleic and linolenic acids de-creased, which was also evident from the decreasing io-dine value, and the content of trans fatty acids increased[127]. A rising content of conjugated fatty acids was ob-served [69, 127]. At that time no great concern was initiat-ed among food scientists and nutritionists. Changes dur-ing deodorization of rapeseed and soybean oils werestudied in the temperature range of 180–240 °C [128] andisomerization of linolenic acid was observed, the rate increasing with temperature. An isomerization degree ofup to 35% was reported at the highest temperature.Changes of soybean oil were studied at still higher tem-peratures of 240–300 °C [129] when not only isomeriza-tion but also polymerization and interesterification tookplace. These last reactions were found to be negligibleonly at lower temperatures of 220–250 °C [97]. The cis,trans isomerization did not take place at all at low temper-atures of only 190 °C [130]. Under common industrialpractice in the USA in the early nineties the content ofgeometrical isomers in refined soybean and canola oilswas 0.3–3.3% octadecadienoic acids, 6.6–37.1% octade-catrienoic acids in the respective fractions, which corre-sponds to 0.56–4.2 trans isomers in total fatty acids [131].Small amounts of cyclic acids with 5–6 member cycleswere detected in heated dienoic and trienoic acid-rich oils[132]. The content of trans isomers in the product wasfound to depend on the fatty acid composition and on the deodorization temperature more than on the time orcontaminants present [133]. In model experimentsdegummed and bleached rapeseed oil was heated at210, 220, and 230 °C for up to 86 hours (not applied in the industrial production) under reduced pressure andwith nitrogen stripping [134, 135]. Under such conditionsthe destruction of linolenic acid was a function of time andtemperature and could be estimated with the use of reac-tion equations. No perceptible isomerization was expect-ed under conventional deodorization conditions. Highercontent of contaminants, which is improbable under nor-mal conditions, would, however, affect the reaction rate[136]. Small amounts of polymers are produced on heat-ing, e.g. 0.32–2.01% dimers (1.07% on average) werefound in refined vegetable oil [137] and 0.4–1.0% in another study [138]. Minute amounts of dimeric triacyl-glycerols were found even in deodorized relatively low unsaturated olive oil [139]. Rapeseed oil refining wasstudied at 200, 225 and 250 °C for 6 h at 0.13–0.26 kPa.Nearly 4% of the trans isomers (mostly cis,cis,trans- andtrans,cis,cis-octadecatrienoic acids) were reported in oilheated to 250 °C [140]. More data on linolenic acid iso-merization as a function of time and temperature can beobtained from companies offering software and equip-ment for physical refining.

Not only fatty acids but also sterols are affected during thephysical refining. They are dimerized by formation of ei-ther linkages or dehydrated, e.g. 3,5-cholestadiene wasfound in refined lard [141], and 24-ethyl-cholesta-2,4,6-triene or 2,4,6,22-tetraene and 24-methyl-2,4,6-trienewere found in refined oils [142].

3.3 Other methods of physical refining

Application of superheated steam was reviewed in the preceding chapter, but the deodorization may beachieved by sparging oil with nitrogen instead of steam.Up to 82% free fatty acids were removed at 162–288 °C[143], and the quality was claimed as better than that ofsteam refined oil. Beef tallow and some vegetable oilswere refined using a stream of nitrogen at 50–270 °C and0.013–0.79 kPa [144]. A suitable equipment was pro-posed, sparkling with nitrogen at 220–240 °C and0.2–0.4 kPa [145]. Nitrogen is more expensive, of course,than steam.

Another – also rather expensive – procedure for strippingfree fatty acids from edible oils is the use of a wiped filmevaporator, which was found successful for the refining oflauric oils, palm oil, or rice bran oil [146]. Stage [147] pro-posed a continuous countercurrent falling film apparatus;99% free fatty acids were removed by using the process.A pilot plant device (annual capacity of 500 t) was foundadvantageous for high price oils, particularly for pharma-ceutical purposes [148]. Temperatures in the range of130–189 °C and pressures of up to 0.2 kPa may be usedto remove fatty acids [149]. The evaporator provided witha thin film dropping worked at 181–200 °C and pressuresup to 1 kPa [150].

Supercritical carbon dioxide degumming already hasbeen discussed, but free fatty acids may be removed bythe same principle [59]. Free fatty acids are soluble at47 °C and at 20 MPa inversely related to their chain lengthand directly related to unsaturation. Isopropyl alcohol issuitable as a cosolvent. The procedure was found appli-cable for rice bran oil and cottonseed oil with relativelyhigh initial acid values [151]. For plant scale edible oil re-fining the process is too expensive.

The membrane technology has been already discussedas suitable but expensive for oil degumming. A similarprocedure could be used for free fatty acid removal [53,54] and for refining milk fat [152]. A combination of a hy-drophilic and a hydrophobic membrane was proposed[153]. In a hexane solution containing 20% oil and 2% ole-ic acid, simulating a miscella, the content of fatty acidscould be reduced by 40% [154]. In another proposal free fatty acids were extracted from soybean oil withmethanol, afterwards separated from the solvent by

Eur. J. Lipid Sci. Technol. 102 (2000) 472–486 Physical refining of edible oils 481

nanofiltration (membrane filtration) using a suitable mem-brane, and the solvent could be recycled [155]. The pro-cedure does not involve heating to high temperatures sothat it could be applied to deacidification of fish oil [156]without the danger of the isomerization of the fish oil fattyacids. The procedure could become commercially appli-cable in future.

3.4 Removal of cholesterol

Cholesterol belongs to objectionable components of ani-mal fats because of their negative effect in the develop-ment of cardiovascular diseases. Therefore, methods for the removal of cholesterol from animal fats weresearched for. The removal is rather expensive, but is stillacceptable as cholesterol-free fats are considered as fatsfor special dietary purposes. Most methods are based onphysical principles. They will be discussed only briefly be-cause cholesterol removal is a very special field of physi-cal refining.

The molecular distillation at 190–250 °C was proposed foran almost complete removal of cholesterol from lard[157]. The method can be applied to butter oil and for theformation of cholesterol-free cream. Another possibility is the extraction of cholesterol with ethanol; about 87.7%cholesterol were removed by triple extraction [158]. Menhaden oil or other fish oils can be separated into alow cholesterol and a high cholesterol fraction, which arestripped with superheated steam [159]; 65% cholesterolwere removed at 204 °C and 0.13 kPa. Cholesterol formsadducts with β-cyclodextrins, and about 90% cholesterolcan be removed in one step [160, 161]. An interesting procedure has been developed at Cargill Co. [162].Processed fat, such as beef tallow, is mixed with lecithinand water and stirred. Cholesterol present in the fatphase is included into phospholipid bilayers formed in thepresence of water. The bilayers are afterwards removedwith the aqueous phase by centrifugation. Cholesterol reduced and cholesterol enriched fractions are thus obtained. The cholesterol content can be decreased byabout 50%. Other procedures for the cholesterol removalexist, but they are based on other principles than utilizedby physical processes. Various procedures were re-viewed by Hayes [163]. Cholesterol-free animal fats aresuitable as a trans acid free component in shortenings.

4 Conclusions

Physical refining consisting of efficient degumming anddeodorization/deacidification has made great progress inthe last 10 years and further developments can be ex-pected. The physical refining procedures are commonlyused on a large scale while being continuously improved.The choice of the optimum technology depends on the

quality of the crude oils, processing equipment, traditionand experience, environmental and economical factors,and quality requirements for the refined products. Amongother methods both the enzyme and membrane technolo-gies are in the stage of a promising development. Cho-lesterol removal will probably soon develop into an indus-trial method to produce animal fats better applicable foredible purposes.

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[Received: May 4, 2000; accepted: June 19, 2000]